AUCTeX is an extensible package for writing and formatting TeX files
in GNU Emacs and XEmacs. It supports many different TeX macro
packages, including AMS-TeX, LaTeX, Texinfo, ConTeXt, and docTeX (dtx
files).

AUCTeX includes preview-latex which makes LaTeX a tightly integrated
component of your editing workflow by visualizing selected source
chunks (such as single formulas or graphics) directly as images in the
source buffer.

Emacs Speaks Statistics (ESS) is an add-on package for emacs-based
text editors such as GNU Emacs and XEmacs. It is designed to support
editing of scripts and interaction with various statistical analysis
programs such S-Plus, R, SAS, Stata, and others. Although all users of
these statistical analysis programs are welcome to apply ESS, advanced
users or professionals who regularly work with text-based statistical
analysis scripts, with various statistical languages/programs, or with
different operating systems might benefit from it the most.

Emacs Muse is an authoring and publishing environment for Emacs. It
simplifies the process of writings documents and publishing them to
various output formats. Muse uses a very simple Wiki-like format as
input.

Emacs has support for spell checking on demand
(ispell) and as your type (flyspell). Both packages depend on a copy
of ispell 3.2 or a compatible spell-checking program. GNU Aspell is a
popular choice these days, and is included in this package.